The present invention relates to a system for a four-wheel drive vehicle for automatically changing the transmission system of the vehicle from two-wheel drive to four-wheel drive in accordance with driving conditions.
In a conventional four-wheel drive vehicle disclosed in Japanese laid open 55-4292, a power transmission system for the two-wheel driving is selectively converted to four-wheel driving by engaging a clutch which is manually operated by a select lever.
For example, when the vehicle travels on slippery roads by two-wheel driving, the transmission system should be changed to the four-wheel drive system in order to prevent the wheels from slipping. If the changing operation is done after the slipping occurs because of the driver's misjudgement of the slipping, the slipping cannot be stopped or reduced.
On the other hand, four-wheel driving under steady driving conditions causes noise. To resolve such unfavorable problems, an automatic control system for the four-wheel drive system has been proposed. A conventional system is provided with a slip detecting circuit which produces a clutch signal when the difference between the speed of the front and rear wheels exceeds a predetermined reference value. The clutch signal causes a clutch to engage, so that the two-wheel drive is automatically changed to the four-wheel drive.
However, the four-wheel driving is useful to drive the vehicle under conditions other than driving on slippery roads. For example, during rapid starting or rapid acceleration, four-wheel driving is effective for preventing slipping which occurs according to driving conditions. Further, during rapid deceleration, four-whel driving is useful to prevent the slipping of the non-driving wheels. In addition, since the engine braking acts on all the wheels, the vehicle is effectively and safely braked.